As small secondary batteries, there have been conventionally known, for example, lithium ion secondary batteries, rechargeable nickel-cadmium batteries, nickel metal hydride batteries, small sealed lead storage batteries, and the like. Among these, lithium ion secondary batteries, because of their high operating voltages, capacities, and energy densities, have been put into practical use mainly as power sources for driving portable electronic devices such as cellular phones, notebook personal computers, video camcorders, and digital still cameras, and there has been an increasing demand for such lithium ion secondary batteries.
In lithium ion secondary batteries, for the positive electrode active material, a lithium-containing composite oxide having a hexagonal crystal structure such as LiCoO2 and LiNiO2, a lithium-containing composite oxide having a spinel structure such as LiMn2O4, and other lithium-containing composite oxides have been mainly used. By using such lithium-containing composite oxides, lithium ion secondary batteries having a 4V-class high-voltage are provided. In particular, by using LiCoO2, lithium ion secondary batteries having a further improved operating voltage and energy density are provided. For this reason, LiCoO2 has been predominantly used as the positive electrode active material.
For the negative electrode active material, carbon materials capable of absorbing and desorbing lithium ions have been used. Among such carbon materials, a graphite material has been predominantly used as the negative electrode active material. A graphite material is suitable for providing lithium ion secondary batteries having a flat discharge potential and a high capacity density. As such, by using LiCoO2 and a graphite material in combination, it is possible to provide a lithium ion secondary battery having both a high capacity and a high energy density and being suitable for use in small-sized consumer apparatus.
In recent years, the technological development for making such lithium ion secondary batteries suitable not only for use in small-sized consumer apparatus but also for use in electric power storage apparatus, electric vehicles, and the like, has been accelerated. In particular, in the field of hybrid electric vehicles (hereinafter referred to as “HEVs”), vehicles in practical use of mass production type powered by a gasoline engine and a nickel metal hydride battery have been developed and are now commercially available. HEVs have been developed for the purpose of reducing the amount of carbon dioxide emission, which is the major cause of global warming. In order to achieve this purpose at a high level and to further improve the power performance and safety performance of the HEVs, with regard to the secondary batteries serving as one of driving power sources for HEVs, a further improvement in performance is required.
Under these circumstances, as a substitute for nickel metal hydride batteries, lithium ion secondary batteries for HEVs have been developed at a rapid pace, and some of such lithium ion secondary batteries have been already put into practical use. In addition, it is predicted that fuel cell-powered vehicles using an output power from a fuel cell to drive the motor will be widely used in the future. For such fuel cell-powered vehicles, secondary batteries having a high output and input and a long service life are indispensable in order to assist the fuel cell, and therefore, high expectation has been placed on lithium ion secondary batteries.
It is necessary for batteries serving as driving power sources for electric vehicles such as HEVs and fuel cell-powered vehicles to instantaneously provide power-assist to the gasoline engine or motor (output) or instantaneously regenerate energy (input), with a given capacity. For this reason, lithium ion secondary batteries to be used for this application are expected to have a high output/input that is considerably much higher than that of lithium ion secondary batteries for use in small-sized consumer apparatus. In order to achieve a higher output/input of the battery, it is effective to reduce the internal resistance of the battery, for which various studies have been made with regard to the electrode structure, the battery components, the electrode active material, the electrolyte, and the like. For example, effective in reducing the internal resistance of the battery are an improvement of the current collecting structure of the electrode, an increase of the reaction area in the electrode by using a thinner and longer electrode, a preparation of a battery component using a less resistive material, and the like.
Further, effective in improving the output/input performance of lithium ion secondary batteries in a low temperature environment is a selection and modification of the electrode active material. In particular, a carbon material used as the negative electrode active material has a great influence on the ability of the negative electrode of absorbing and desorbing lithium ions, and thus on the output/input performance of the battery. This means that by using a carbon material being highly capable of absorbing and desorbing lithium as the negative electrode active material, a lithium ion secondary battery having a high output/input performance can be obtained.
When used in small-sized consumer apparatus, lithium ion secondary batteries are required to have a high capacity and a high energy density. Accordingly, in such lithium ion secondary batteries, with the priority put on the capacity and energy density, LiCoO2 (positive electrode active material) and a graphite material (negative electrode active material) are used in combination; however, since the output/input performance of the graphite material is not sufficiently high, this combination cannot be predominant in the electric vehicle application and the like that require higher output and input. As for the carbon material serving as the negative electrode active material, in the case of using a carbon material that is not sufficiently graphitized rather than a highly crystalline graphite material, a high output/input performance can be obtained, although the battery capacity is slightly reduced. For such a carbon material, various proposals have been made.
For example, one proposal suggests a graphitizable carbon material of which the wide-angle X-ray diffraction pattern measured with CuKα radiation shows a ratio [I(101)/I(100)] of a peak intensity I(101) attributed to a (101) plane to a peak intensity I(100) attributed to a (100) plane exceeds 0 and is less than 1.0 (see, e.g., Patent Document 1). The carbon material of Patent Document 1 is obtained by heating a coke material at about 1800° C. to 2200° C. to partially graphitize the coke material.
The grain size of the crystallite of this carbon material is comparatively small, and the crystal structure of this carbon material contains a large proportion of turbostratic structure in which a graphitized region and a non-graphitized region are co-present. As such, this carbon material is excellent in the output/input performance because of easy absorption and desorption of lithium ions thereto and therefrom and quick diffusion of lithium ions therein. However, with regard to the output/input performance during charge and discharge in a low temperature zone, a further improvement is expected.
Another proposal suggests a negative electrode active material having a bilayer structure comprising a core material of graphite powder and a coating layer provided on the surface of the core material, the coating layer made of a low crystalline carbon (see, e.g., Patent Document 2). The coating layer is formed by coating the surface of the graphite powder with a carbon precursor and heating the graphite powder with the carbon precursor in an inert gas atmosphere at a temperature of 700 to 2800° C. to carbonize the carbon precursor. Here, the carbon precursor is, as disclosed in paragraph [0018] of Patent Document 2, coal-tar pitch, various heavy oils, heat-treated pitch, vinyl-based resin, formaldehyde-based resin, aromatic hydrocarbon, nitrogen-containing heterocyclic compound, sulfur-containing heterocyclic compound, and the like.
The carbon precursor is converted into a low crystalline carbon through carbonization. Although the negative electrode active material of Patent Document 2 has a bilayer structure, the graphite material occupies the major portion thereof. Using a graphite material makes it easy to reliably achieve a high energy density; however, since graphite materials have large crystallites and thus has a large anisotropy, using a graphite material is not suitable for absorbing and desorbing lithium ions quickly. For this reason, with regard to the negative electrode active material of Patent Document 2 also, a further improvement in the output/input performance is expected.
Yet another proposal suggests a negative electrode active material having a bilayer structure comprising a core material being a composite of graphite and hard carbon and a coating layer provided on the surface of the core material, the coating layer made of a low crystalline carbon (see, e.g., Patent Document 3). Here, the hard carbon is a non-graphitizable carbon, which is prepared by carbonizing at 1000 to 1400° C. a non-meltable fiber being a by-product of the production of a carbon fiber, or alternatively carbonizing at 1000 to 1400° C. an oxide obtained by oxidizing an organic raw material in air at 150 to 300° C.
The organic raw material is coal- or petroleum-based isotropic pitch, phenolic resin, furan resin, furfural resin, and the like. Since the negative electrode active material of Patent Document 3 also contains graphite, resulted from the crystalline structure of graphite, the reaction of absorbing and desorbing lithium ions proceeds most slowly in a low temperature zone. Accordingly, the output/input performance in a low temperature zone of the negative electrode active material of Patent Document 3 is not in a fully satisfactory level.
In order to facilitate the reaction of absorbing and desorbing lithium ions particularly in a low temperature zone of 0° C. or lower, it is desirable to use a low-crystalline graphitizable carbon having a crystal structure containing a large proportion of turbostratic structure. However, the capacity density of the low-crystalline graphitizable carbon is 200 Ah/kg or less and the initial efficiency thereof is as low as 90% or less. Because of this, it is difficult to provide a higher energy density. For this reason, the low-crystalline graphitizable carbon is not suitable as a negative electrode active material for a lithium ion battery.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-335132    Patent Document 2: Japanese Patent Publication No. 3193342    Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 11-246209